3 Answers
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The limit for IRA contributions is $5,500 per year for both 2016 and 2017. If you wanted to you could deposit $11,000 all at once into your IRA account between now and April 15, and allocate 5500 to each year. This is true regardless of whether it is a Traditional or Roth IRA. However, since you already filed your 2016, it is easiest to put the 2016 amount into a Roth, otherwise you would have to amend your 2016 return. Note you have until April 15, 2018 to make all of your 2017 IRA contributions.

Tip: if you want to deposit more than 5500 prior to April 15 all at once, I would recommend splitting it up into two transactions (5500 for 2016 and however much for 2017). This is because many banks allocate a deposit to the tax year, and may not be able to split a single deposit in their system for reporting purposes.